Light modulated signals in an optical communication system typically deteriorate depending on the integrity of its system components. For example, an optical signal transmitted over a channel of a fiber cable can be distorted due to imperfections in the cable itself. In certain applications, distortion of an optical signal is detectable after traveling over just a few feet of fiber cable. This type of signal distortion is often exacerbated when an optical signal is transmitted over a long haul such as thousands of feet of fiber cable.
Fiber optic cables and other optical communication components can distort optical signals depending on the wavelength over which the signals are transmitted. Consider an optical system in which multiple optical signals are transmitted over separate channels, each of which is defined by a particular wavelength. Based on the characteristics of the communication system and its components, certain optical signals can be attenuated or distorted more than others depending on the wavelength at which the optical signal is transmitted. This can result in an uneven spectral profile. Consequently, optical signals received at an end of a long cable can have different signal power levels and different signal to noise ratios, which can impair the performance of an optical communication system.
In addition to fiber cables, another component typically used in an optical communication system that can negatively effect a spectral profile is Raman scattering. The spectral profile in a fiber Raman Amplifier can vary from xe2x88x920.08 dB/nm to 0.08 dB/nm. Using a low pump power setting and a long fiber, an overall Raman gain is typically biased towards longer wavelength channels due to the accumulation of Raman gain produced by the signal channels. Using high pump power and a shorter fiber cable, shorter wavelength channels typically experience a higher gain than longer wavelength channels. As a result, a spectral profile of multiple optical signals of differing wavelengths can become skewed so that the spectral profile of an optical communication system is no longer flat.
A feature of the present invention relates to a fixed or dynamic optical filter device to control an optical input at a single wavelength or multiple wavelengths.
One aspect of the present invention is generally directed towards an apparatus and method for filtering an optical input. In an illustrative embodiment, an optical input is split into polarization components along separate paths. The polarization components are then fed into a first solid state electro-optic device that includes electrodes across which a voltage is applied to adjust a wavelength transmission characteristic of the device. A section of the first electro-optic device positioned between the electrodes preferably has a birefringence that is adjusted depending on the voltage applied across the electrodes. The adjusted components are thereafter combined to produce an optical output. Accordingly, an optical input can be attenuated based on a voltage applied to electrodes of the first solid state device.
In one application, the first solid state device is used to filter one or multiple optical signals and the voltage applied across electrodes of the device is adjusted to attenuate a selected range of wavelengths of the optical input to produce an optical output. For example, the voltage applied across the electrodes of the first electro-optic device is controlled to adjust the phase difference of the separate polarization components at certain wavelengths. The adjusted components along separate paths are then recombined to produce an optical output signal that is generally an attenuated version of the optical input signal at selected wavelengths depending on the applied voltage.
Although the first solid state device for adjusting retardation of the input signal components can be made of almost any suitable material providing an electro-optic effect, at least a portion of the body of the first electro-optic device can be chosen from ceramic type of material such as PLZT, PMN, PMN-PT, or PLMNT. Likewise, the material can be chosen from crystal materials such as LiNbO3, LiTaO3 or PLMNT. For example, the first electro-optic device can be fabricated from a block of any such material, or combination of materials, to which an electric field can be applied. Preferably, electrodes are disposed directly on the block of material to apply the electric field. However, other methods of applying an electric field can be used in accordance with the principles of the present invention. In addition to materials similar to that previously mentioned, the body of the first electro-optic device additionally can be fabricated from a polymer material having birefringent characteristics.
In one application, the electro-optic device is fabricated using a liquid crystal material according to the principles of the present invention.
As previously discussed, a section of the electro-optic device disposed between corresponding electrodes has a variable birefringence that can be adjusted by controlling a voltage applied across the electrodes. In one application, the first electro-optic device additionally includes a section of material having a fixed birefringence. The section of fixed birefringence can be adjacently positioned relative to the section of variable birefringence to receive the polarization components along separate paths.
In yet another application, a second solid-state electro-optic device is also positioned to receive the polarization components transmitted along separate paths. The second electro-optic device preferably includes electrodes across which a second voltage is applied to adjust a polarization orientation of the optical components. Consequently, adjusting the polarization orientation of the separated optical components in this way adjusts a degree to which the optical input is attenuated at a particular wavelength to produce an output signal.
The second electro-optic device can be an optical retarder device sandwiched between two quarterwave plates.
Optionally, a Faraday rotator and a corresponding electro-magnet device are provided in lieu of the second solid state device to adjust the polarization orientation of the optical input signal components. While a voltage applied across the first electro-optic device adjusts one or multiple wavelengths at which the optical input signal is attenuated, the Faraday rotator generally can be used to adjust the degree to which selected wavelengths are attenuated.
Another aspect of the present invention involves providing an attenuation profile for filtering a given optical input. Typically, the attenuation profile of the filter is approximately sinusoidal over a frequency range. Amplitude characteristics of an attenuation profile can be controlled by adjusting the second voltage across the electrodes of the second electro-optic device.
A phase of an attenuation profile for filtering the optical input signal can be adjusted. For example, the attenuation profile also can be shifted or changed across a range of wavelengths by adjusting the first voltage applied across the electrodes of the first electro-optic device.
When both the first and second electro-optic devices are used in a common application, both phase and amplitude characteristics of an attenuation profile can be adjusted based on corresponding voltages applied to the first and second electrodes. Additionally, multiple serially disposed filter stages, each including a combination of electro-optic devices, can be dynamically adjusted so that an optical output signal has a desired profile across a range of wavelengths. For instance, it can be particularly useful to dynamically adjust an output signal so that it has a flat spectral profile across a range of wavelengths.